* [merged mm-stable] documentation-mm-update-dax-vmemmap-deduplication-docs.patch removed from -mm tree
@ 2026-10-09 5:59 Andrew Morton
0 siblings, 0 replies; only message in thread
From: Andrew Morton @ 2026-10-09 5:59 UTC (permalink / raw)
To: mm-commits, sshegde, rppt, ritesh.list, rdunlap, qi.zheng,
osalvador, npiggin, mpe, maddy, ljs, lance.yang, david, corbet,
chleroy, muchun.song, akpm
The quilt patch titled
Subject: Documentation/mm: update DAX vmemmap deduplication docs
has been removed from the -mm tree. Its filename was
documentation-mm-update-dax-vmemmap-deduplication-docs.patch
This patch was dropped because it was merged into the mm-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
------------------------------------------------------
From: Muchun Song <muchun.song@linux.dev>
Subject: Documentation/mm: update DAX vmemmap deduplication docs
Date: Wed, 30 Sep 2026 22:06:27 +0800
Device DAX now uses the common per-zone shared tail page for vmemmap
deduplication. The old documentation still described a DAX-specific
layout with a separately populated tail vmemmap page and half the HugeTLB
savings.
Update the generic and powerpc documentation to describe the shared
layout. In the powerpc document, keep the radix and 64K-specific details,
drop the duplicated 4K PUD arithmetic, and replace the repeated device-dax
diagrams with a single parameterized PMD/PUD diagram.
Link: https://lore.kernel.org/20260930140627.57431-13-songmuchun@bytedance.com
Signed-off-by: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <chleroy@kernel.org>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Lance Yang <lance.yang@linux.dev>
---
Documentation/arch/powerpc/vmemmap_dedup.rst | 90 +++--------------
Documentation/mm/vmemmap_dedup.rst | 32 ------
2 files changed, 21 insertions(+), 101 deletions(-)
--- a/Documentation/arch/powerpc/vmemmap_dedup.rst~documentation-mm-update-dax-vmemmap-deduplication-docs
+++ a/Documentation/arch/powerpc/vmemmap_dedup.rst
@@ -19,82 +19,28 @@ With 1G PUD level mapping, we require 16
vmemmap page can contain 1024 struct pages (64K/sizeof(struct page)). Hence we
require 16 64K pages in vmemmap to map the struct page for 1G PUD level mapping.
-Here's how things look like on device-dax after the sections are populated::
- +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
- | | | 0 | -------------> | 0 |
- | | +-----------+ +-----------+
- | | | 1 | -------------> | 1 |
- | | +-----------+ +-----------+
- | | | 2 | ----------------^ ^ ^ ^ ^ ^
- | | +-----------+ | | | | |
- | | | 3 | ------------------+ | | | |
- | | +-----------+ | | | |
- | | | 4 | --------------------+ | | |
- | PUD | +-----------+ | | |
- | level | | . | ----------------------+ | |
- | mapping | +-----------+ | |
- | | | . | ------------------------+ |
- | | +-----------+ |
- | | | 15 | --------------------------+
- | | +-----------+
- | |
- | |
- | |
- +-----------+
-
-
With 4K page size, 2M PMD level mapping requires 512 struct pages and a single
4K vmemmap page contains 64 struct pages(4K/sizeof(struct page)). Hence we
require 8 4K pages in vmemmap to map the struct page for 2M pmd level mapping.
-Here's how things look like on device-dax after the sections are populated::
-
- +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
- | | | 0 | -------------> | 0 |
- | | +-----------+ +-----------+
- | | | 1 | -------------> | 1 |
- | | +-----------+ +-----------+
- | | | 2 | ----------------^ ^ ^ ^ ^ ^
- | | +-----------+ | | | | |
- | | | 3 | ------------------+ | | | |
- | | +-----------+ | | | |
- | | | 4 | --------------------+ | | |
- | PMD | +-----------+ | | |
- | level | | 5 | ----------------------+ | |
- | mapping | +-----------+ | |
- | | | 6 | ------------------------+ |
- | | +-----------+ |
- | | | 7 | --------------------------+
- | | +-----------+
- | |
- | |
- | |
- +-----------+
-
-With 1G PUD level mapping, we require 262144 struct pages and a single 4K
-vmemmap page can contain 64 struct pages (4K/sizeof(struct page)). Hence we
-require 4096 4K pages in vmemmap to map the struct pages for 1G PUD level
-mapping.
-
-Here's how things look like on device-dax after the sections are populated::
-
- +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
- | | | 0 | -------------> | 0 |
- | | +-----------+ +-----------+
- | | | 1 | -------------> | 1 |
- | | +-----------+ +-----------+
- | | | 2 | ----------------^ ^ ^ ^ ^ ^
- | | +-----------+ | | | | |
- | | | 3 | ------------------+ | | | |
- | | +-----------+ | | | |
- | | | 4 | --------------------+ | | |
- | PUD | +-----------+ | | |
- | level | | . | ----------------------+ | |
- | mapping | +-----------+ | |
- | | | . | ------------------------+ |
- | | +-----------+ |
- | | | 4095 | --------------------------+
- | | +-----------+
+Here's how things look on device-dax after vmemmap-optimized sections are
+populated. ``N`` is the number of vmemmap pages required by the DAX mapping
+above::
+
+ Device DAX vmemmap pages (N pages) backing page frames
+ +-----------+ ---virt_to_page---> +-----------+ mapping to +-------------+
+ | | | 0 | -------------> | 0 |
+ | | +-----------+ +-------------+
+ | | | 1 | ------+
+ | | +-----------+ |
+ | | | 2 | ------+
+ | | +-----------+ |
+ | | | . | ------+ +-------------+
+ | PMD/PUD | +-----------+ | | A single, |
+ | level | | . | ------+------> | per-zone |
+ | mapping | +-----------+ | | shared tail |
+ | | | N - 1 | ------+ | page |
+ | | +-----------+ +-------------+
| |
| |
| |
--- a/Documentation/mm/vmemmap_dedup.rst~documentation-mm-update-dax-vmemmap-deduplication-docs
+++ a/Documentation/mm/vmemmap_dedup.rst
@@ -1,4 +1,3 @@
-
.. SPDX-License-Identifier: GPL-2.0
=========================================
@@ -192,32 +191,7 @@ to 4 on HugeTLB pages.
There's no remapping of vmemmap given that device-dax memory is not part of
System RAM ranges initialized at boot. Thus the tail page deduplication
-happens at a later stage when we populate the sections. HugeTLB reuses the
-the head vmemmap page representing, whereas device-dax reuses the tail
-vmemmap page. This results in only half of the savings compared to HugeTLB.
-
-Deduplicated tail pages are not mapped read-only.
+happens at a later stage when we populate the sections.
-Here's how things look like on device-dax after the sections are populated::
-
- +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
- | | | 0 | -------------> | 0 |
- | | +-----------+ +-----------+
- | | | 1 | -------------> | 1 |
- | | +-----------+ +-----------+
- | | | 2 | ----------------^ ^ ^ ^ ^ ^
- | | +-----------+ | | | | |
- | | | 3 | ------------------+ | | | |
- | | +-----------+ | | | |
- | | | 4 | --------------------+ | | |
- | PMD | +-----------+ | | |
- | level | | 5 | ----------------------+ | |
- | mapping | +-----------+ | |
- | | | 6 | ------------------------+ |
- | | +-----------+ |
- | | | 7 | --------------------------+
- | | +-----------+
- | |
- | |
- | |
- +-----------+
+Deduplicated tail pages are not mapped read-only. The mapping layout is the same
+as HugeTLB.
_
Patches currently in -mm which might be from muchun.song@linux.dev are
mm-sparse-vmemmap-drop-vmemmap_populate_dax.patch
mm-sparse-vmemmap-support-device-dax-in-common-vmemmap-path.patch
mm-sparse-vmemmap-drop-device-dax-specific-population-path.patch
mm-sparse-vmemmap-remove-the-unused-ptpfn-argument.patch
mm-sparse-vmemmap-open-code-vmemmap_populate_address.patch
mm-mm_init-add-zone-mismatch-warning-during-page-init.patch
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2026-10-09 5:59 [merged mm-stable] documentation-mm-update-dax-vmemmap-deduplication-docs.patch removed from -mm tree Andrew Morton
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